Semiconductor device, fabrication method of the semiconductor devices

ABSTRACT

In a semiconductor device, a YAG substrate is formed as a single-crystal substrate of any of surface orientations (100), (110), and (111). In the fabrication of the semiconductor device, a TMAl gas is first fed onto the YAG substrate so as to form a nucleation layer made of aluminum, which is a group-III element. Then, an NH 3  gas is fed onto the nucleation layer. This turns the surface of the nucleation layer into a group-V element and then forms a group-III-V compound layer of AlN. Then, a mixed gas of TMAl gas and NH 3  gas is fed onto the group-III-V compound layer so as to form another group-III-V compound layer. Finally, a group-III nitride semiconductor layer is crystal-grown on the group-III compound layer.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Application No. 2009-221212, filed on Sep. 25,2009, the entire content of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device and a method forfabricating the semiconductor devices and, more particularly, to asemiconductor device having a substrate made of Y₃Al₅O₁₂ and a group-IIInitride semiconductor layer and a fabrication method thereof.

2. Description of the Related Art

Recent years have seen a spate of active development of technologiesusing phosphor materials designed to obtain light emitting modules foremitting white light, for instance, using a light emitting device suchas an LED (light emitting diode). For example, it is possible to producewhite light by placing a phosphor material, which can be excited by bluelight to emit yellow light, on an LED that emits blue light. In thisregard, Japanese Unexamined Patent Application Publication No. 2006-5367(Reference (1) in the Related Art List below) proposes a structurehaving a ceramic layer disposed within the path of light emitted from alight emitting layer, for instance.

For example, Reference (1) discloses bonding as a way of attaching aphosphor material, which has been formed into a sheet-like ceramic layeror the like, to a light emitting layer. However, the bonding layer isliable to degradation from exposure to the light from the light emittinglayer. Also, the bonding layer can develop voids therewithin, and thepresence of voids may cause a decrease in the light extractionefficiency. And, provision of a bonding layer whose refractive index isrelatively low may also cause a decrease in the light extractionefficiency. Also, since the light transmissibility of the bonding layeris lower than 100%, the light passing through the bonding layer maypossibly contribute to a decline in the light extraction efficiency.Further, a step of bonding is required aside from the step of allowingcrystal growth of a semiconductor layer on a growth substrate. Moreover,there is need for an expensive substrate of sapphire, SiC, or the likefor crystal growth thereon in addition to the phosphor material, whichhas been formed into a sheet-like ceramic layer or the like.

As a measure to solve the above-mentioned drawbacks, the above-citedpatent publication of Reference (1) proposes a technology in which agroup-III nitride nucleation layer is deposited at low temperaturedirectly on a ceramic layer and further a buffer layer of GaN (galliumnitride) is deposited at high temperature thereon. According to thispatent publication, it is possible to eliminate harmful influences oflattice mismatch by inserting multiple low-temperature intermediatelayers between YAGs (see below) and the GaN buffer layers. Also,Japanese Unexamined Patent Application Publication No. 2003-204080(Reference (2) in the Related Art List below), for instance, proposes anitride semiconductor device in which a buffer layer is formed on asubstrate formed of Y₃Al₅O₁₂ of surface orientation (111) (hereinafterreferred to as YAG (Yttrium Aluminum Garnet) as appropriate) and agroup-III nitride semiconductor layer is formed on the buffer layer.

RELATED ART LIST

-   (1) Japanese Unexamined Patent Application Publication No.    2006-5367.-   (2) Japanese Unexamined Patent Application Publication No.    2003-204080.

However, as disclosed in the first-cited Reference (1), many steps mustbe taken if multiple layers are to be deposited on the ceramic layerbefore the growth of a light emitting layer. There is thus room forimprovement in productivity in the fabrication of the light emittingmodule. Also, the technology disclosed in the second-cited Reference (2)assumes the use of a substrate formed of YAG of surface orientation(111). Yet, YAG exists in surface orientations other than (111) as well,and there is also much demand for the use of YAG of polycrystallinestructure in this field of art.

SUMMARY OF THE INVENTION

The present invention has been made to solve problems as describedabove, and a purpose thereof is to provide a technology for forming agroup-III nitride semiconductor layer in simple steps for any of YAGshaving a plurality of surface orientations.

To resolve the foregoing problems, a method, for fabricating asemiconductor device, according to one embodiment of the presentinvention comprises: forming a buffer layer, containing a group-III-Vcompound, on a substrate formed of Y₃Al₅O₁₂; and forming a group-IIInitride semiconductor layer on the buffer layer. The forming the bufferlayer includes forming a nucleation layer made of a group-III element onthe substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described by way of examples only, withreference to the accompanying drawings which are meant to be exemplary,not limiting and wherein like elements are numbered alike in severalFigures in which:

FIG. 1 is a cross-sectional view of a semiconductor device according toa first embodiment of the present invention;

FIG. 2 is a diagram showing the feed timing of TMAl gas and NH₃ gas whenforming a buffer layer according to a first embodiment of the presentinvention;

FIGS. 3A to 3D illustrate fabrication processes of a semiconductordevice according to a first embodiment of the present invention;

FIG. 4A is a diagram showing the feed timing of TMAl gas and NH₃ gaswhen fabricating a semiconductor device of Comparative Example 1;

FIG. 4B is a photo of a surface of a semiconductor device of ComparativeExample 1;

FIG. 4C is a photo of a cross section of a semiconductor device ofComparative Example 1;

FIG. 5A is a diagram showing the feed timing of TMAl gas and NH₃ gaswhen forming a buffer layer of a first embodiment;

FIG. 5B is a photo of a surface of a semiconductor device of a firstembodiment;

FIG. 5C is a photo of a cross section of a semiconductor device of afirst embodiment;

FIG. 6A is a diagram showing the feed timing of TMAl gas and NH₃ gaswhen forming a buffer layer of a semiconductor device of ComparativeExample 2;

FIG. 6B is a photo of a surface of a semiconductor device of ComparativeExample 2;

FIG. 6C is a photo of a cross section of a semiconductor device ofComparative Example 2;

FIG. 7A is a diagram showing the feed timing of TMAl gas and NH₃ gaswhen forming a buffer layer of a semiconductor device of ComparativeExample 3;

FIG. 7B is a photo of a surface of a semiconductor device of ComparativeExample 3;

FIG. 7C is a photo of a cross section of a semiconductor device ofComparative Example 3;

FIG. 8 is a diagram showing the feed timing of TMAl gas and NH₃ gas whenforming a buffer layer of a semiconductor device of Comparative Example4;

FIG. 9A is a photo of a surface of a semiconductor device of ComparativeExample 4;

FIG. 9B is a photo of a cross section of a semiconductor device ofComparative Example 4;

FIG. 10A is a photo of a surface of a semiconductor device of a firstembodiment, for which a buffer layer was formed with the feed timing ofTMAl gas and NH₃ gas as shown in FIG. 2 and the pulse feeding time t1 ofTMAl gas of 5 seconds;

FIG. 10B is a photo of a cross section of a semiconductor device in thecase of FIG. 10A;

FIG. 11A is a photo of a surface of a semiconductor device of a firstembodiment, for which a buffer layer was formed with the feed timing ofTMAl gas and NH₃ gas as shown in FIG. 2 and the pulse feeding time t1 ofTMAl gas of 10 seconds;

FIG. 11B is a photo of a cross section of a semiconductor device in thecase of FIG. 11A;

FIG. 12A is a photo of a surface of a semiconductor device of a firstembodiment, for which a buffer layer was formed with the feed timing ofTMAl gas and NH₃ gas as shown in FIG. 2 and the pulse feeding time t1 ofTMAl gas of 15 seconds;

FIG. 12B a photo of a cross section of a semiconductor device in thecase of FIG. 12A;

FIG. 13A is a photo of a surface of a semiconductor device according toa first embodiment, for which a buffer layer was formed with the feedtiming of TMAl gas and NH₃ gas as shown in FIG. 2 when the pulse feedingtime t1 of TMAl gas is 20 seconds;

FIG. 13B a photo of a cross section of a semiconductor device 10 in thecase of FIG. 13A;

FIG. 14 is a diagram showing a relationship between the pulse feedingtime t1 of TMAl gas (horizontal axis) and the half-value width in GaN(0002) rocking curve measurement (vertical axis) of a semiconductordevice according to a first embodiment;

FIG. 15 is a diagram showing a relationship between the pulse feedingtime t1 of TMAl gas (horizontal axis) and the half-value width in GaN(10-12) rocking curve measurement (vertical axis) of a semiconductordevice according to a first embodiment;

FIG. 16 is a photo of a surface of a semiconductor device of ComparativeExample 4 using a YAG substrate of surface orientation (111);

FIG. 17A is a photo of a surface of a semiconductor device according toa first embodiment using a YAG substrate of surface orientation (111);

FIG. 17B a photo showing a bird's eye view of a cross section of thesemiconductor device of FIG. 17A;

FIG. 18 is a photo of a surface of a semiconductor device of ComparativeExample 5 using a YAG substrate of surface orientation (110);

FIG. 19A is a photo of a surface of a semiconductor device according toa first embodiment using a YAG substrate of surface orientation (110);

FIG. 19B is a photo of a cross section of the semiconductor device ofFIG. 19A;

FIG. 20 is a photo of a surface of a semiconductor device of ComparativeExample 6 using a YAG substrate of surface orientation (100);

FIG. 21A is a photo of a surface of a semiconductor device according toa first embodiment using a YAG substrate of surface orientation (100);

FIG. 21B is a photo showing a bird's eye view of a cross section of thesemiconductor device of FIG. 21A;

FIG. 22 is a cross-sectional view of a semiconductor device according toa second embodiment of the present invention;

FIG. 23 is a diagram showing the feed timing of TMAl gas and NH₃ gaswhen forming a buffer layer of a semiconductor device according to asecond embodiment of the present invention;

FIGS. 24A to 24C illustrate fabrication processes of a semiconductordevice according to a second embodiment of the present invention;

FIG. 25 is a cross-sectional view of a semiconductor device according toa third embodiment of the present invention;

FIG. 26 is a diagram showing the feed timing of TMAl gas and NH₃ gaswhen forming a buffer layer of a semiconductor device according to athird embodiment of the present invention; and

FIG. 27A to 27C illustrate fabrication processes of a semiconductordevice according to a third embodiment of the present invention

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be described by reference to the preferredembodiments. This does not intend to limit the scope of the presentinvention, but to exemplify the invention. Hereinbelow, the embodimentswill now be described in detail with reference to drawings.

First Embodiment

FIG. 1 is a cross-sectional view of a semiconductor device 10 accordingto a first embodiment of the present invention. The semiconductor device10 includes a YAG substrate 12, a buffer layer 14, and a group-IIInitride semiconductor layer 16.

The YAG substrate 12 is what is generally called a light emittingceramic or a fluorescent ceramic. It can be produced by sintering aceramic green body made from the powder of YAG (Yttrium AluminumGarnet), or Y₃Al₅O₁₂, which is a fluorescent material capable of beingexcited by blue light. The YAG substrate 12 thus produced emits yellowlight by converting a wavelength of blue light. Hence, a semiconductordevice 10 that emits white light as a synthesized light of blue andyellow through an additive color mixing can be formed by combining theYAG substrate 12 with a semiconductor layer emitting blue light.

In the first embodiment, the YAG substrate 12 is formed into a plateshape. Also, the YAG substrate 12 to be employed is a single-crystalsubstrate of any of surface orientations (100), (110), and (111) or apolycrystalline substrate of a plurality of surface orientations.

The YAG substrate 12 may be such that part of Y of Y₃Al₅O₁₂ is replacedby any of Lu, Sc, La, Gd, and Sm and/or part of Al is replaced by any ofIn, B, Tl, and Ga. Also, the YAG substrate 12 may contain at least onetype of activator selected from the group consisting of Ce, Tb, Eu, Ba,Sr, Mg, Ca, Zn, Si, Cu, Ag, Au, Fe, Cr, Pr, Nd, Dy, Co, Ni, and Ti. Itis to be noted that Ce may be Ce³⁺, Tb may be Tb²⁺, and Eu may be Eu²⁺.

Also, the YAG substrate 12 is so formed as to be transparent. In thefirst embodiment, the term “transparent” or “transparency” should beunderstood to mean that the total light transmission in the convertedwavelength range is 40% or above. As a result of diligent R&D efforts bythe inventors, it has been found that as long as the transparency is ata total transmission of light in the converted wavelength range of 40%or above, the wavelength of light can be properly converted by the YAGsubstrate 12 and the decrease in light emitted from the YAG substrate 12can be controlled properly. Therefore, with the YAG substrate 12 havinga transparency as described above, the light emitted by thesemiconductor layer can be converted efficiently.

Also, the YAG substrate 12 is formed of an inorganic material, which isfree of any organic binder, and is therefore more durable than when itcontains organic materials such as organic binder. Consequently, anelectric power of 1 watt or above, for instance, can be applied to thelight emitting module, thereby raising the brightness, luminosity, andluminous flux of the light to be produced by the light emitting module.

The buffer layer 14, which contains a group-III-V compound, is formed onthe YAG substrate 12. The buffer layer 14 functions as a relaxationlayer to effect the single crystal growth of a group-III nitridesemiconductor layer 16. Therefore, it is formed on the YAG substrate 12before the single crystal growth of the group-III nitride semiconductorlayer 16.

The buffer layer 14 has a nucleation layer 18 and a group-III-V compoundlayer 20. For the semiconductor device 10 according to the firstembodiment, the nucleation layer 18 is formed on the surface of the YAGsubstrate 12 before the group-III-V compound layer 20 is formed, so thatthe group-III nitride semiconductor layer 16 can be formed properlywhatever YAG substrate 12 having a plurality of surface orientations isused.

The nucleation layer 18 is first formed of aluminum on the YAG substrate12. It should be appreciated here that the nucleation layer 18 may beformed of another group-III element. The nucleation layer 18 may also beformed of one or more of Al, Ga, and In as the group-III elements.

In the first embodiment, the surface of the nucleation layer 18 isturned into a group-V compound by a group-V element. As a result, thenucleation layer 18 includes a group-III nucleation layer 22 left therewithout being combined with the group-V element and a group-III-Vcompound layer 24 with the surface of the nucleation layer 18, which wasonce formed of the group-III element, now formed by being combined witha group-V element.

The group-III-V compound layer 20 is formed on the nucleation layer 18.The group-III nitride semiconductor layer 16 is formed through a processof single crystal growth on the surface of the thus formed buffer layer14.

FIG. 2 is a diagram showing the feed timing of TMAl (trimethylaluminum)gas and NH₃ gas when forming the buffer layer 14 according to the firstembodiment. In the first embodiment, the buffer layer 14 is formed by aMOCVD (metal organic chemical vapor deposition) process. FIG. 2 showsthe feed timing of the TMAl gas and the feed timing of the NH₃ gas inthis MOCVD process. FIGS. 3A to 3D illustrate the fabrication processesof a semiconductor device 10 according to the embodiment of FIG. 1.Hereinbelow, a method for fabricating the semiconductor device 10 isexplained in conjunction with FIG. 2 and FIGS. 3A to 3D.

As shown in FIG. 2, in the first embodiment, the TMAl gas is first fedin pulses onto the YAG substrate 12 without allowing the NH₃ gas to beexposed to the surface of the YAG substrate 12. Let t1 represent thispulse feeding time. When time t2 has elapsed from the end of pulsefeeding of the TMAl gas, the feeding of the NH₃ gas is started. Whentime t3 has elapsed from the start of the NH₃ gas feeding, the feedingof the TMAl gas is started again. This will supply a mixed gas of TMAlgas and NH₃ gas onto the YAG substrate 12. Let t4 represent the feedingtime of TMAl gas at this time. Note that either or both of H₂ and N₂ areused as the carrier gas for the TMAl gas and the carrier gas for NH₃gas.

FIGS. 3A to 3D illustrate the fabrication processes of a semiconductordevice 10 according to the first embodiment of the present invention.FIG. 3A is an illustration showing a state in which a nucleation layer18 has been formed on the surface of a YAG substrate 12. In the firstembodiment, the semiconductor device 10 is fabricated such that thenucleation layer 18, which is made of aluminum, is formed on the YAGsubstrate 12 by pulse-feeding a TMAl gas containing aluminum, which is agroup-III element, onto the surface of the YAG substrate 12 for a pulsefeeding time t1. It is to be noted that the nucleation layer 18 may beformed by another film-forming method such as sputtering or vacuumdeposition.

[FIG. 3B is an illustration showing a state in which the surface of thenucleation layer 18 has been nitrided. After an elapse of t2 from theend of pulse feeding of the TMAl gas, an NH₃ gas is now fed onto thenucleation layer 18. By this step, the surface of the nucleation layer18 is nitrided, which results in the formation of a group-III-V compoundlayer 24 by changing the surface of the nucleation layer 18 into AlN(aluminum nitride), which is a group-III-V compound. And the part thathas remained aluminum without being nitrided is a group-III nucleationlayer 22. It should be noted that nitridation of the nucleation layer 18may be kept up until there is no longer group-III nucleation layer 22remaining. Also, at least a part of the surface of the nucleation layer18 may be combined with a group-V element by feeding a gas containing agroup-V element other than nitrogen onto the nucleation layer 18.

FIG. 3C is an illustration showing a state in which a group-III-Vcompound layer 20 has been formed on the nucleation layer 18. When timet3 has elapsed from the start of NH₃ gas feeding, the feeding of TMAlgas is started again. In this manner, a mixed gas of TMAl gas containingaluminum, which is a group-III element, and NH₃ gas containing nitrogen,which is a group-V element, is fed onto the nucleation layer 18, so thatthe group-III-V compound layer 20 of AlN is stacked on the nucleationlayer 18.

Thus a buffer layer 14 is constituted by the nucleation layer 18 and thegroup-III-V compound layer 20. It should be noted that a plurality ofbuffer layers 14 may also be formed between the YAG substrate 12 and thegroup-III nitride semiconductor layer 16 by forming another buffer layer14 on top of the buffer layer 14.

FIG. 3D is an illustration showing a state in which a group-III nitridesemiconductor layer 16 has been formed on the buffer layer 14. Thegroup-III nitride semiconductor layer 16 is formed by growing thecrystal of In_(x)Al_(y)Ga_((1−x−y))N (0≦x≦1, 0≦y≦1, 0≦x+y≦1) on thebuffer layer 14 using an epitaxial growth method. The group-III nitridesemiconductor layer 16 is provided such that it emits light covering atleast a part of the wavelength range when a voltage is applied.Specifically, In_(x)Al_(y)Ga_((1−x−y))N is first doped with an n-typeimpurity so as to allow a semiconductor layer to grow on the bufferlayer 14. Through this process, an n-type semiconductor layer is formedon the buffer layer 14. Next, the In_(x)Al_(y)Ga_((1−x−y))N is dopedwith a p-type impurity so as to further grow the semiconductor layerthereon. It is to be noted here that a quantum well light emitting layermay be provided between the n-type semiconductor layer and the p-typesemiconductor layer. Also, ELO (epitaxial lateral overgrowth) may beemployed as the epitaxial growth method.

MOCVD is used to carry out the crystal growth of these semiconductorlayers. However, it is evident to those skilled in the art that thecrystal growth technique is not limited to MOCVD, but an MBE (molecularbeam epitaxy) method may be used for the crystal growth of thosesemiconductor layers.

After this, a part of the p-type semiconductor layer is removed byetching to expose a part of the upper surface of the n-typesemiconductor layer. Electrodes are formed on the exposed upper surfaceof the n-type semiconductor layer and the upper surface of the p-typesemiconductor layer. Finally, the prepared stack is diced intosemiconductor devices 10 of proper size. In the first embodiment, thedicing produces semiconductor devices 10 of 1 mm rectangle. Thegroup-III nitride semiconductor layer 16 thus formed functions as asemiconductor light emitting device when a voltage is applied thereto.The first embodiment improves productivity in the fabrication of lightemitting modules by eliminating the step of bonding the YAG substrate 12to the group-III nitride semiconductor layer 16. Also, the firstembodiment can reduce cost by doing away with the expensive sapphiresubstrate or SiC substrate.

Comparative Example 1

FIG. 4A is a diagram showing the feed timing of TMAl gas and NH₃ gaswhen fabricating a semiconductor device of Comparative Example 1. Thesemiconductor device of Comparative Example 1 is fabricated by the samemethod as for the semiconductor device 10 according to the firstembodiment with the exception of the method for forming the bufferlayer.

In Comparative Example 1, the TMAl gas is not pulse-fed to the surfaceof the YAG substrate 12, and the NH₃ gas is not fed thereto, either. InComparative Example 1, a mixed gas of TMAl gas and NH₃ gas is fed to thesurface of the YAG substrate 12. The feeding time of the TMAl gas atthis time, represented by t11, is 600 seconds in Comparative Example 1.In this manner, a buffer layer is formed on the YAG substrate 12, andthe crystal of the group-III nitride semiconductor layer 16 is grownfurther thereon.

[FIG. 4B is a photo of a surface of the semiconductor device ofComparative Example 1, and FIG. 4C a photo of a cross section of thesemiconductor device of Comparative Example 1. As is clear from FIG. 4Band FIG. 4C, the surface of the GaN layer, which is the group-IIInitride semiconductor layer 16 of Comparative Example 1, is uneven.

FIG. 5A is a diagram showing the feed timing of TMAl gas and NH₃ gaswhen forming a buffer layer 14 of the first embodiment. When thesemiconductor device 10 was fabricated for comparison with ComparativeExample 1, t1 was 15 seconds, t2 was one minute, t3 was 5 minutes, andt4 was 600 seconds.

FIG. 5B is a photo of a surface of the semiconductor device 10 of thefirst embodiment, and FIG. 5C a photo of a cross section of thesemiconductor device 10 of the first embodiment. As is clear from FIG.5B and FIG. 5C, there is little unevenness visible on the surface of theGaN layer of the semiconductor device 10, indicating that the crystalgrowth of the group-III nitride semiconductor layer 16 has been betterdone than Comparative Example 1.

Comparative Example 2

FIG. 6A is a diagram showing the feed timing of TMAl gas and NH₃ gaswhen forming a buffer layer of a semiconductor device of ComparativeExample 2. The semiconductor device of Comparative Example 2 is alsofabricated by the same method as for the semiconductor device 10according to the first embodiment with the exception of the method forforming the buffer layer.

In Comparative Example 2, the surface of the YAG substrate 12 is firstexposed to NH₃ gas for t21. In Comparative Example 2, t21 is 5 minutes.When t22 has elapsed after the end of exposure to NH₃ gas, a mixed gasof TMAl gas and NH₃ gas is fed onto the YAG substrate 12 for t23. InComparative Example 2, t22 is one minute and t23 is 600 seconds.

FIG. 6B is a photo of a surface of the semiconductor device ofComparative Example 2, and FIG. 6C a photo of a cross section of thesemiconductor device of Comparative Example 2. As is clear from FIG. 6Band FIG. 6C, the exposure of the surface of the YAG substrate 12 to NH₃gas before a mixed gas of TMAl gas and NH₃ gas is fed thereto results ingreater unevenness of the surface of the GaN layer than the case shownin FIG. 4B and FIG. 4C which does not involve the exposure of thesurface of the YAG substrate 12 to NH₃ gas. This indicates that thesurface of the YAG substrate 12 should not be exposed to NH₃ gas if thegroup-III nitride semiconductor layer 16 is to be formed properly.

Comparative Example 3

FIG. 7A is a diagram showing the feed timing of TMAl gas and NH₃ gaswhen forming a buffer layer of a semiconductor device of ComparativeExample 3. In Comparative Example 3, NH₃ gas is first fed to the surfaceof the YAG substrate 12 for t31. After a short time from the end of thisexposure to the NH₃ gas, TMAl gas is now pulse-fed for t32. After theend of pulse feeding of the TMAl gas, NH₃ gas only is fed for t33. Whent33 has elapsed from the start of feeding NH₃ gas, the feeding of TMAlgas is started and a mixed gas of TMAl gas and NH₃ gas is fed for t34.In Comparative Example 3, t31 is 5 minutes, t32 is 15 seconds, t33 is 5minutes, and t34 is 600 seconds.

FIG. 7B is a photo of a surface of the semiconductor device ofComparative Example 3, and FIG. 7C a photo of a cross section of thesemiconductor device of Comparative Example 3. As is clear from FIG. 7Band FIG. 7C, the TMAl gas pulse-fed even after the exposure of thesurface of the YAG substrate 12 to NH₃ gas reduces the unevenness of thesurface of the GaN layer substantially from the case shown in FIG. 6Band FIG. 6C.

Comparative Example 4

FIG. 8 is a diagram showing the feed timing of TMAl gas and NH₃ gas whenforming a buffer layer of a semiconductor device of Comparative Example4. An attempt was made to find an optimum time for t2 as shown in FIG. 2by observing changes in the surface of the GaN layer with changes in t2.At the same time, they were compared with the surface of the GaN layerof Comparative Example 4.

In Comparative Example 4, NH₃ gas is first fed to the surface of the YAGsubstrate 12 for t41. After that, the feeding of TMAl gas is started anda mixed gas of TMAl gas and NH₃ gas is fed to the YAG substrate 12 fort42. In Comparative Example 4, t41 is 30 seconds, and t42 is 600seconds.

FIG. 9A is a photo of a surface of the semiconductor device ofComparative Example 4, and FIG. 9B a photo of a cross section of thesemiconductor device of Comparative Example 4. As is clear from FIG. 9Aand FIG. 9B, the surface of the GaN layer shows a marked unevenness.

FIG. 10A is a photo of a surface of the semiconductor device 10according to the first embodiment, for which the buffer layer was formedwith the feed timing of TMAl gas and NH₃ gas as shown in FIG. 2 and thepulse feeding time t1 of TMAl gas of 5 seconds, and FIG. 10B a photo ofa cross section of the semiconductor device 10 in this case. As otherconditions, t3 and t4 of FIG. 2 were 5 minutes, and 600 seconds,respectively.

FIG. 11A is a photo of a surface of the semiconductor device 10according to the first embodiment, for which the buffer layer was formedwith the feed timing of TMAl gas and NH₃ gas as shown in FIG. 2 and thepulse feeding time t1 of TMAl gas of 10 seconds, and FIG. 11B a photo ofa cross section of the semiconductor device 10 in this case. As for thetiming, t3 and t4 of FIG. 2 are the same as those for the fabricationprocess of the semiconductor device shown in FIG. 10A and FIG. 10B.

FIG. 12A is a photo of a surface of the semiconductor device 10according to the first embodiment, for which the buffer layer was formedwith the feed timing of TMAl gas and NH₃ gas as shown in FIG. 2 and thepulse feeding time t1 of TMAl gas of 15 seconds, and FIG. 12B a photo ofa cross section of the semiconductor device 10 in this case. As for thetiming, t3 and t4 of FIG. 2 are the same as those for the fabricationprocess of the semiconductor device shown in FIG. 10A and FIG. 10B.

FIG. 13A is a photo of a surface of the semiconductor device 10according to the first embodiment, for which the buffer layer was formedwith the feed timing of TMAl gas and NH₃ gas as shown in FIG. 2 when thepulse feeding time t1 of TMAl gas is 20 seconds, and FIG. 13B a photo ofa cross section of the semiconductor device 10 in this case. As for thetiming, t3 and t4 of FIG. 2 are the same as those for the fabricationprocess of the semiconductor device shown in FIG. 10A and FIG. 10B.

When the pulse feeding time t1 is so changed as to be 5 seconds, 10seconds, 15 seconds, and 20 seconds as mentioned above, the surfaceunevenness of the GaN layer is reduced to a minimum when the pulsefeeding time t1 is 15 seconds as is evident in FIG. 12A and FIG. 12B. Ithas thus been confirmed that the advantageous effect of the nucleationlayer 18 being formed of aluminum is the most marked when t1 is 15seconds.

FIG. 14 is a diagram showing a relationship between the pulse feedingtime t1 of TMAl gas (horizontal axis) and the half-value width in GaN(0002) rocking curve measurement (vertical axis) of the semiconductordevice 10 according to the first embodiment. The pulse feeding time t1was changed from zero seconds to 60 seconds.

As is clear from FIG. 14, the results of GaN (0002) rocking curvemeasurement show the smallest value of diffraction peak half-value widthwhen the pulse feeding time t1 of TMAl gas is 15 seconds. This indicatesthat the GaN layer was formed optimally when the pulse feeding time t1of TMAl gas was 15 seconds.

FIG. 15 is a diagram showing a relationship between the pulse feedingtime t1 of TMAl gas (horizontal axis) and the half-value width in GaN(10-12) rocking curve measurement (vertical axis) of the semiconductordevice 10 according to the first embodiment. The pulse feeding time t1was changed from zero seconds to 60 seconds.

As is clear from FIG. 15, the results of GaN (10-12) rocking curvemeasurement also show the smallest value of diffraction peak half-valuewidth when the pulse feeding time t1 of TMAl gas is 15 seconds. Thisindicates that the GaN layer was formed optimally when the pulse feedingtime t1 of TMAl gas was 15 seconds.

FIG. 16 is a photo of a surface of the semiconductor device ofComparative Example 4 using a YAG substrate of surface orientation(111). FIG. 16 is a representation at a low magnification of the surfacephoto of Comparative Example 4 shown in FIG. 9A so as to facilitatecomparison with other semiconductors. As is evident, the surface of theGaN layer shows a marked unevenness.

FIG. 17A is a photo of a surface of the semiconductor device 10according to the first embodiment using a YAG substrate of surfaceorientation (111), and FIG. 17B a photo showing a bird's eye view of across section of the semiconductor device 10. As already described, forthe semiconductor device 10 of the first embodiment, a GaN layer isformed after the formation of the buffer layer by feeding TMAl gas andNH₃ gas onto the YAG substrate with the feed timing as shown in FIG. 2.The pulse feeding time t1 of TMAl gas is 15 seconds, which have beenconfirmed to produce optimal results. As is clear from FIG. 17A and FIG.17B, when the YAG substrate of surface orientation (111) is used, thepulse feeding of TMAl gas leads to a substantial reduction of surfaceunevenness of the GaN layer.

Comparative Example 5

FIG. 18 is a photo of a surface of a semiconductor device of ComparativeExample 5 using a YAG substrate of surface orientation (110). Thefabrication method of the semiconductor device is the same as that ofComparative Example 4 except that a YAG substrate of surface orientation(110) is used. Hence, for the semiconductor device of ComparativeExample 5, the GaN layer is formed after the formation of the bufferlayer by feeding TMAl gas and NH₃ gas onto the YAG substrate with thefeed timing as shown in FIG. 8. As for the timing, t41 and t42 are thesame as those for Comparative Example 4. Thus, the semiconductor deviceof Comparative Example 5 also shows a marked surface unevenness of theGaN layer.

FIG. 19A is a photo of a surface of the semiconductor device 10according to the first embodiment using a YAG substrate of surfaceorientation (110), and FIG. 19B a photo of a cross section of thesemiconductor device 10. The fabrication method of the semiconductordevice shown in FIGS. 19A and 19B is the same as that of thesemiconductor device shown in FIGS. 17A and 17B except that a YAGsubstrate of surface orientation (110) is used. The pulse feeding timet1 of TMAl gas for this example is also 15 seconds, which have beenconfirmed to produce optimal results when the YAG substrate of surfaceorientation (111) is used.

As is clear from FIG. 19A and FIG. 19B, when the YAG substrate ofsurface orientation (110) is used, the pulse feeding of TMAl gas alsoleads to a substantial reduction of surface unevenness of the GaN layer.

Comparative Example 6

FIG. 20 is a photo of a surface of a semiconductor device of ComparativeExample 6 using a YAG substrate of surface orientation (100). Thefabrication method of the semiconductor device is the same as that ofComparative Example 4 except that a YAG substrate of surface orientation(100) is used. Hence, for the semiconductor device of ComparativeExample 6, the GaN layer is formed after the formation of a buffer layerby feeding TMAl gas and NH₃ gas onto the YAG substrate with the feedtiming as shown in FIG. 8. As for the timing, t41 and t42 are the sameas those for Comparative Example 4. Thus, the semiconductor device ofComparative Example 6 also shows a marked surface unevenness of the GaNlayer.

FIG. 21A is a photo of a surface of the semiconductor device 10according to the first embodiment using a YAG substrate of surfaceorientation (100), and FIG. 21B a photo showing a bird's eye view of across section of the semiconductor device 10. The fabrication method ofthe semiconductor device shown in FIGS. 21A and 21B is the same as thatof the semiconductor device shown in FIGS. 17A and 17B except that theYAG substrate of surface orientation (100) is used. The pulse feedingtime t1 of TMAl gas for this example is also 15 seconds, which have beenconfirmed to produce optimal results when the YAG substrate of surfaceorientation (111) is used.

As is clear from FIG. 21A and FIG. 21B, when the YAG substrate ofsurface orientation (100) is used, the pulse feeding of TMAl gas alsoleads to a substantial reduction of surface unevenness of the GaN layer.However, a streaky unevenness remains on the surface of the GaN layer.Under the present conditions, therefore, it has been found that the 15seconds of pulse feeding of TMAl gas produce better results when the YAGsubstrate of surface orientation (110) or (111) is used than when thatof surface orientation (100) is used.

As described above, according to the semiconductor 10 of the firstembodiment, the GaN layer, which is a group-III nitride semiconductorlayer 16, can be formed properly not only when the YAG substrate ofsurface orientation (111) is used, but also when the YAG substrate ofsurface orientation (110) or (100) is used.

Second Embodiment

FIG. 22 is a cross-sectional view of a semiconductor device 100according to a second embodiment of the present invention. Hereinbelow,the structural components identical to those of the first embodiment aregiven the identical reference numerals, and the repeated descriptionthereof will be omitted as appropriate.

The semiconductor device 100 includes a YAG substrate 12, a buffer layer102, and a group-III nitride semiconductor layer 16. The buffer layer102, which contains a group-III-V compound, is formed on the YAGsubstrate 12. The buffer layer 102 functions as a relaxation layer toeffect the single crystal growth of the group-III nitride semiconductorlayer 16.

The buffer layer 102 has a nucleation layer 104 and a group-III-Vcompound layer 20. For the semiconductor device 100 according to thesecond embodiment, the nucleation layer 104 is formed on the surface ofthe YAG substrate 12 before the group-III-V compound layer 20 is formed,so that the group-III nitride semiconductor layer 16 can be formedproperly whatever YAG substrate 12 having a plurality of surfaceorientations is used.

The nucleation layer 104 is first formed of aluminum on the YAGsubstrate 12. It should be appreciated here that the nucleation layer104 may be formed of another group-III element. The nucleation layer 104may also be formed of one or more of Al, Ga, and In as the group-IIIelements. The group-III-V compound layer 20 is formed on the nucleationlayer 104. The group-III nitride semiconductor layer 16 is formedthrough a process of single crystal growth on the surface of the thusformed buffer layer 106.

FIG. 23 is a diagram showing the feed timing of TMAl gas and NH₃ gaswhen forming the buffer layer 102 of the semiconductor device 100according to the second embodiment of the present invention. In thesecond embodiment, too, the buffer layer 104 is formed by a MOCVDprocess.

In the second embodiment, the TMAl gas is first fed in pulses onto theYAG substrate 12 without allowing the NH₃ gas to be exposed to thesurface of the YAG substrate 12. Let t51 represent this pulse feedingtime. When time t52 has elapsed from the end of pulse feeding of theTMAl gas, the feeding of both the TMAl gas and the NH₃ gas is started.This will supply a mixed gas of TMAl gas and NH₃ gas onto the YAGsubstrate 12. Let t53 represent the feeding time of TMAl gas at thistime. The pulse feeding time t51 may be 15 seconds, whereas the feedingtime t53 may be 600 seconds.

FIGS. 24A to 24C illustrate fabrication processes of the semiconductordevice 100 according to the second embodiment of the present invention.FIG. 24A is an illustration showing a state in which a nucleation layer104 has been formed on the surface of a YAG substrate 12. In the secondembodiment, the semiconductor device 100 is fabricated such that thenucleation layer 104 made of aluminum is formed on the YAG substrate 12by pulse-feeding a TMAl gas containing aluminum, which is a group-IIIelement, onto the surface of the YAG substrate 12 for a pulse feedingtime t51. It is to be noted that the nucleation layer 104 may be formedby another film-forming method such as sputtering or vacuum deposition.

FIG. 24B is an illustration showing a state in which the group-III-Vcompound layer 20 is formed on the nucleation layer 104. After an elapseof t52 from the end of pulse feeding of the TMAl gas, a mixed gas ofTMAl gas and NH₃ gas is fed, so that the group-III-V compound layer 20of AlN is stacked on the nucleation layer 18. Thus a buffer layer 102 isconstituted by the nucleation layer 104 and the group-III-V compoundlayer 20.

FIG. 24C is an illustration showing a state in which the group-III-Vcompound layer 20 has been formed on the buffer layer 102. The group-IIInitride semiconductor layer 16 is formed by epitaxially growing thecrystal of GaN on the buffer layer 102. As described above, thegroup-III-V compound layer 20 is directly formed on the nucleation layer104 made of a group-III element. In the second embodiment, too, apreferable group-III nitride semiconductor layer 16 with little surfaceunevenness is formed.

Third Embodiment

FIG. 25 is a cross-sectional view of a semiconductor device 150according to a third embodiment of the present invention. Hereinbelow,the structural components identical to those of the first embodiment aregiven the identical reference numerals, and the repeated descriptionthereof will be omitted as appropriate.

The semiconductor device 150 includes a YAG substrate 12, a buffer layer152, and a group-III nitride semiconductor layer 16. The buffer layer152, which contains a group-III-V compound, is formed on the YAGsubstrate 12. The buffer layer 152 functions as a relaxation layer toeffect the single crystal growth of the group-III nitride semiconductorlayer 16.

The buffer layer 152 has a group-III nucleation layer 154 and agroup-III-V compound layer 156. For the semiconductor device 150according to the third embodiment, a nucleation layer made of aluminum,which is a group-III element, is first formed on the surface of the YAGsubstrate 12, so that the group-III nitride semiconductor layer 16 canbe formed properly whatever YAG substrate 12 having a plurality ofsurface orientations is used. The nucleation layer may also be formed ofone or more of Al, Ga, and In as the group-III elements. The surface ofthe nucleation layer is combined with a group-V element, and thegroup-III-V compound layer 156 is formed on the surface of thenucleation layer. A partial region of the nucleation layer left therewithout being combined with the group-V element becomes the group-IIInucleation layer 154. Note that the entire nucleation layer may becombined with a group-V element and therefore no group-III nucleationlayer 154 may be left there. Thereby, the buffer layer 152 isconstituted by the group-III nucleation layer 154 and the group-III-Vcompound layer 156. The group-III nitride semiconductor layer 16 isformed through a process of single crystal growth on the surface of thethus formed buffer layer 152.

FIG. 26 is a diagram showing the feed timing of TMAl gas and NH₃ gaswhen forming the buffer layer 152 of the semiconductor device 150according to the third embodiment of the present invention. In the thirdembodiment, the TMAl gas is first fed in pulses onto the YAG substrate12 without allowing the NH₃ gas to be exposed to the surface of the YAGsubstrate 12. Let t61 represent this pulse feeding time. When time t62has elapsed from the end of pulse feeding of the TMAl gas, the NH₃ gasis fed for time t63. As a result, the surface of the nucleation layer154 is combined with a group-V element, and the group-III-V compoundlayer 156 is formed on the surface the nucleation layer 154. The pulsefeeding time t61 may be 15 seconds, whereas the feeding time t63 may be5 minutes.

FIG. 27A to 27C illustrate fabrication processes of the semiconductordevice 150 according to the third embodiment of the present invention.FIG. 27A is an illustration showing a state in which a nucleation layer158 has been formed on the surface of a YAG substrate 12. In the thirdembodiment, the semiconductor device 150 is fabricated such that thenucleation layer 158 made of aluminum is formed on the YAG substrate 12by pulse-feeding a TMAl gas containing aluminum, which is a group-IIIelement, onto the surface of the YAG substrate 12 for a pulse feedingtime t61. It is to be noted that the nucleation layer 158 may be formedby another film-forming method such as sputtering or vacuum deposition.

FIG. 27B is an illustration showing a state in which the surface of thenucleation layer 158 is changed to the group-III compound layer 156after it has been combined with a group-V element. After an elapse oft62 from the end of pulse feeding of the TMAl gas, the NH₃ gas is fedonto the nucleation layer 158 and thereby the surface of the nucleationlayer 158 is turned into a group-V compound by a group-V element; as aresult, the group-III-V compound layer 156 of AlN is formed. Thus thebuffer layer 152 is constituted by the group-III-V compound layer 156and the group-III nucleation layer 154, which is a partial region leftwithout being combined with a group-V element. Note that the entirenucleation layer may be combined with a group-V element and therefore nogroup-III nucleation layer 154 may be left there.

FIG. 27C is an illustration showing a state in which the group-IIIcompound layer 16 has been formed on the buffer layer 152. The group-IIInitride semiconductor layer 16 is formed by epitaxially growing thecrystal of GaN on the buffer layer 152. As described above, thegroup-III-V compound layer 156 is formed in such a manner that thesurface of the nucleation layer 158 is turned into the group-IIIcompound layer 156 after it has been combined with a group-V element. Inthis third embodiment, too, a preferable group-III nitride semiconductorlayer 16 with little surface unevenness is formed.

The present invention is not limited to each of the above-describedembodiments only, and those resulting from any combination of therespective elements as appropriate are effective as embodiments.

Also, it is understood by those skilled in the art that variousmodifications such as changes in design may be added to each of theembodiments based on their knowledge and the embodiments added with suchmodifications are also within the scope of the present invention.

1. A method for fabricating a semiconductor device, the methodcomprising: forming a buffer layer, containing a group-III-V compound,on a substrate formed of Y₃Al₅O₁₂; and forming a group-III nitridesemiconductor layer on the buffer layer, wherein the forming the bufferlayer includes forming a nucleation layer made of a group-III element inat least a part on the substrate.
 2. A method for fabricating asemiconductor device according to claim 1, wherein the nucleation layeris formed by supplying a first gas containing a group-III element ontothe substrate.
 3. A method for fabricating a semiconductor deviceaccording to claim 1, wherein the forming a buffer layer furtherincludes changing at least a part of the surface of the nucleation layerinto a group-III-V compound by combining the at least a part of thesurface of the nucleation layer with a group V element.
 4. A method forfabricating a semiconductor device according to claim 3, wherein the atleast a part of the surface of the nucleation layer is changed into agroup-III-V compound by supplying a second gas containing a group-Velement onto the nucleation layer.
 5. A method for fabricating asemiconductor device according to claim 1, wherein the forming a bufferlayer further includes forming a group-III-V compound layer on thenucleation layer.
 6. A method for fabricating a semiconductor deviceaccording to claim 5, wherein the group-III-V compound layer is formedon the nucleation layer by supplying a third gas containing both of agroup-III element and a group-V element onto the nucleation layer.
 7. Amethod for fabricating a semiconductor device according to claim 1,wherein a nucleation layer made of one or more of Al, Ga, and In asgroup-III elements is formed.
 8. A method for fabricating asemiconductor device according to claim 1, further comprising forminganother of the buffer layer on the buffer layer.
 9. A method forfabricating a semiconductor device according to claim 1, wherein thesubstrate is a single-crystal substrate of any of surface orientations(100), (110), and (111) or a polycrystalline substrate of a plurality ofsurface orientations.
 10. A method for fabricating a semiconductordevice according to claim 1, wherein the substrate is such that part ofY is replaced by any of Lu, Sc, La, Gd, and Sm and/or part of Al isreplaced by any of In, B, Tl, and Ga.
 11. A method for fabricating asemiconductor device according to claim 1, wherein the substratecontains at least one type of activator selected from the groupconsisting of Ce, Tb, Eu, Ba, Sr, Mg, Ca, Zn, Si, Cu, Ag, Au, Fe, Cr,Pr, Nd, Dy, Co, Ni, and Ti.
 12. A method for fabricating a semiconductordevice according to claim 11, wherein the activator contains Ce³⁺.
 13. Amethod for fabricating a semiconductor device according to claim 11,wherein the activator contains Tb²⁺.
 14. A method for fabricating asemiconductor device according to claim 11, wherein the activatorcontains Eu²⁺.
 15. A semiconductor device comprising: a substrate formedof Y₃Al₅O₁₂; and a buffer layer, containing a group-III-V compound,formed on the substrate; and a group-III nitride semiconductor layerformed on the buffer layer, the buffer layer including: a nucleationlayer, made of a group-III element, formed on the substrate; and agroup-III-V compound layer formed on the nucleation layer.